Coherent phenomena in semiconductors

A review of coherent phenomena in photoexcited semiconductors is presented. In particular, two classes of phenomena are considered. The first is concerned with the role played by optically induced phase coherence in the ultrafast spectroscopy of semiconductors; the other with the Coulomb-induced effects on the coherent optical response of low-dimensional structures. All the phenomena discussed in this review are analysed in terms of a theoretical framework based on the density-matrix formalism. Due to its generality, this quantum-kinetic approach allows a realistic description of coherent as well as incoherent, i.e. phase-breaking, processes, thus providing quantitative information on the coupled - coherent versus incoherent - carrier dynamics in photoexcited semiconductors. The primary goal of this review is to discuss the concept of quantum-mechanical phase coherence as well as its relevance to and implications for semiconductor physics and technology. In particular, we will discuss the dominant role played by optically induced phase coherence on the process of carrier photogeneration and relaxation in bulk systems. We will then review typical field-induced coherent phenomena in semiconductor superlattices such as Bloch oscillations and Wannier-Stark localization. Finally, we will discuss the dominant role played by Coulomb correlation on the linear and nonlinear optical spectra of realistic quantum-wire structures.